"If it's to be a geological period, it has to be visible in the
geological record," said Anthony Brown, a researcher at the
University of Southampton in the United Kingdom, who is trying to
define the boundary.

Because geology looks at the deep past, such questions would
normally be examined in 100,000, even 1 million years' time, he
said.

"In the absence of
time travel, we have to work out whether we really do have
enough around to define a new geological period."

Shifting boundary

In one possible way of demarcating the boundary between natural
geologic eras and the human-shaped period, scientists would look
at how
agriculture changes sediments, Brown told LiveScience.

For instance, when farmers clear-cut forests and plant crops,
they change how sediments and runoff wash into the local rivers,
often creating a thick layer of silty, sandy clay on the flood
plain, Brown said. [Top
10 Ways to Destroy Earth]

But using such geologic clues to date the Anthropocene era runs
into a problem: agriculture began at different times around the
globe. Some areas, such as certain pockets in Africa, may not
have had intensive agriculture until recently.

Alternatively chemical deposits could date the boundary between
human and natural geologic eras. For instance, widespread use of
leaded gasoline and paint has left
high levels of lead in soils throughout the world, said
Michael Kruge, a researcher at Montclair State University in New
Jersey.

Polycyclic aromatic hydrocarbons (PAH) could also serve as
markers. These are formed from combustion in natural wildfires,
but also come largely from the burning of fossil fuels.

"In the middle of the 20th century, you see a big spike in these
compounds in sediment," Kruge said in a press conference.

Using those measures, the Anthropocene era would begin around
industrialization, thousands of years after humans began
reshaping the planet with agriculture.

Yet another proposal would peg the time of the Anthropocene's
birth to the mass movement of soil, or the accumulation of
minerals from coal burning, cement production for construction,
or the massive use of nitrogen fertilizers. That would date the
Anthropocene to the sharp uptick in the production of these
chemicals after
World War II.

Other scientists hope to date the Anthropocene's onset using
modern-day fossils, for instance, layers and layers of plastic
soda bottles and tin cans piling up in landfills.

All of these approaches face a challenge, however: combining
human-caused changes with natural, global variations that
normally demarcate different geologic time periods. For instance,
our current geologic time period, the Holocene Epoch, governs our
climate and the extent of our glaciers, and is dictated by
eccentricities in the Earth's orbit (something humans' haven't
yet managed to alter).

Since the Earth's orbit isn't going to change any time soon, the
Anthropocene would somehow need to overlap with the Holocene.

"Nobody believes that the astronomical cycle, the 100,000-year
cycle that we're in, is suddenly coming to an end," Brown said.
"We have to combine the anthropogenic with the natural
variability in the climate system. It is a question that
geologists never had to face before."